A plurality of wireless access systems are in use at the present time. 3GPP TS23.401 V12.2.0 and 3GPP TS23.007 V12.2.1, Section 27 pertaining, for example, to a core network called Evolved Packet Core (EPC) in the 3rd Generation Partnership Project (3GPP) have been standardized as technologies for aggregating the plurality of wireless access systems. For example, as illustrated in FIG. 1, a 3G network such as W-CDMA and a LET network are aggregated in an EPC. The EPC includes a PDN gateway (PGW), a serving gateway (SGW), and a mobility management entity (MME) as network devices. FIG. 1 is a view for explaining Evolved Packet Core (EPC). The communication system illustrated in FIG. 1 includes a serving GPRS support node (SGSN) and a radio network controller (RNC) as a 3G network.
The SGW is mainly a gateway for handling user data. As illustrated in FIG. 2 for example, the SGW has a plurality of user plane (U-plane) processing units that use a “communication tunnel” to which user terminal “sessions” are assigned, and a control plane (C-plane) processing unit for managing the communication tunnels. The control plane processing unit sends and receives signals for managing quality of service (QoS) and for following the movements of the users. FIG. 2 is a view of a configuration example of a serving gateway (SGW). A “communication tunnel” is a logical communication path provided between user plane processing units and base stations. The user plane processing units are associated with one or a plurality of communication tunnels. A “session” is information held in the EPC for connecting a user terminal to a public data communication network (PDN), and includes, for example, user terminal IDs and position information of user terminals. Moreover, “setting (establishing) a session” signifies associating a communication tunnel with a session. By setting (establishing) a session, the transfer of user data using a communication tunnel is made possible since the user terminal and the communication tunnel are associated with each other. In FIG. 1, the dotted lines signify a control plane and the solid lines signify a user plane. The ovals #a and #b in FIG. 2 indicate the respective sessions of user terminals #a and #b. Therefore, the session #a of the user terminal #a is established in the user plane processing unit in a blade #1, and the session #b of the user terminal #b is established in the user plane processing unit in a blade #2.
The MME cooperates with the control plane processing unit to control switching of the communication tunnels used for the user terminal communication. The PGW transfers user data between the SWG and an external network (for example, PDN).
Conventionally, the PGW monitors each SGW for defects (failures) in the SGWs. When user data addressed to a first user terminal is received from the external network, the PGW determines whether any defects are present in a first SGW in which the session with the first user terminal is established. If a defect is present, the PGW transmits a “user data notification” to the MME via a second SGW different from the first SGW, and the MME establishes a session with the first user terminal with another SGW other than the first SGW. As a result, even if a SGW in which a session is established has a defect, the user data can be transferred to the user terminal address.